Electrical switching devices include, for example, circuit switching devices and circuit interrupters such as circuit breakers, contactors, motor starters, motor controllers and other load controllers. Circuit breakers are generally old and well known in the art. Examples of circuit breakers are disclosed in U.S. Pat. Nos. 4,606,313; 4,887,057; 5,200,724; and 5,341,191.
Circuit breakers are widely used in industrial, commercial and residential applications for protecting electrical circuitry, conductors and apparatus from damage due to an overcurrent condition, such as an overload condition or a relatively high level short circuit condition. Initially used as a direct replacement for fuses, circuit breakers have been gradually called upon to provide more sophisticated types of protection other than merely interrupting the circuit when the current flow exceeds a certain level. More elaborate time-current trip characteristics have been developed such that a circuit breaker can rapidly open upon very high current with the time delay being roughly inversely proportional to the degree of overload. Circuit breakers are also available which interrupt upon the detection of ground fault currents. As the complexity of electrical distribution circuits has increased, the control portions of the circuit breaker have been augmented to provide appropriate protection and monitoring functions.
During the late 1960's, solid state electronic trip units were developed for use in high power, low voltage circuit breakers. These electronic trip units performed functions such as instantaneous and delayed tripping which were traditionally achieved by magnetic and thermal means. The improved accuracy and flexibility of the solid state electronic trip units resulted in their wide spread acceptance.
The earliest electronic trip unit circuit designs utilized discrete components such as transistors, resistors and capacitors. More recently, designs, such as disclosed in U.S. Pat. No. 4,428,022, have included microprocessors which provide improved performance and flexibility. See also, for example, U.S. Pat. No. 5,525,985.
These digital systems sample the current waveforms periodically to generate a digital representation of the current. The microprocessor uses the samples to execute algorithms which implement the current protection curve which is typically based upon a constant value of I.sup.2 t where "I" is the value of current and "t" is the time-to-trip. Typically, in air circuit breakers and vacuum circuit interrupters, this protection curve provides an instantaneous trip, a long delay trip and, if appropriate, a short delay trip function. In some circuit interrupters, the microprocessor also performs voltage protection as well as calculations for metering such as determining the RMS value of the highest phase current.
It is also common today to have a plurality of such circuit interrupters monitored and controlled by a central network control station such as is described in U.S. Pat. No. 5,420,799. The central monitoring and control network typically communicates digitally with multiple circuit interrupters over a common network. In turn, the circuit interrupters usually have mechanical status indicating accessory devices mounted within the casings which are used by external and remote monitoring and control equipment. Examples include auxiliary contacts which follow the open/close status of the circuit breaker and bell alarm contacts which are closed if the breaker is in the tripped condition.
The increase in functional versatility that the electronic trip units of circuit breakers have added by employing the capabilities of microprocessors has required the processing circuitry to monitor wide ranges in current and related parameters which can vary from the open state, to a normal operating range in which most of the metering calculations are performed, to short circuit states which exceed the normal operating range by a factor of ten times or much greater. In turn, the process required to configure such electronic trip units is very complex and typically requires either the substantial involvement of a user through a programming panel or separate programming device or, alternatively, the use of a host computer or the like to automatically perform such programming.
Accordingly, there is room for improvement in the configuration of trip units and electrical switching devices employing such units.